diff --git a/src/bin/main.ml b/src/bin/main.ml
index 41dbc6a..da71dba 100644
--- a/src/bin/main.ml
+++ b/src/bin/main.ml
@@ -9,16 +9,16 @@ let stop    = ref 30.0
 let greedy  = ref false
 let inplace = ref false
 let steps   = ref 1
-let model   = ref ""
+let model   = ref None
 
 let gt0i v i  = v := if i <= 0   then 1   else i
 let gt0f v f  = v := if f <= 0.0 then 1.0 else f
-let hasmodel  = ref false
 let modelargs = ref []
 
 let set_model s =
-  if !hasmodel then modelargs := s :: !modelargs
-  else model := s; hasmodel := true
+  match !model with
+  | None   -> model := Some s
+  | Some _ -> modelargs := s :: !modelargs
 
 let opts = [
   "-sample",  Arg.Int (gt0i sample), "n \tSample count (default=10)";
@@ -40,11 +40,12 @@ let z = StatefulZ.(Functional.zsolve)
 let s = Solver.solver_c c z
 open Format
 let m = match !model with
-  | "ball"   -> Ball.bouncing_ball
-  | "vdp"    -> Vdp.van_der_pol
-  | "sincos" -> Sincos.sinus_cosinus
-  | "sqrt"   -> Sqrt.sqrt
-  | _        -> eprintf "Unknown model: %s\n" !model; exit 2
+  | None          -> eprintf "Missing model\n"; exit 2
+  | Some "ball"   -> Ball.bouncing_ball
+  | Some "vdp"    -> Vdp.van_der_pol
+  | Some "sincos" -> Sincos.sinus_cosinus
+  | Some "sqrt"   -> Sqrt.sqrt
+  | Some s        -> eprintf "Unknown model: %s\n" s; exit 2
 let m = try m !modelargs with Invalid_argument s -> eprintf "%s\n" s; exit 2
 
 let state = if !inplace then (module State.InPlaceSimState    : State.SimState)